Sheet metal and method of manufacture



Dec. 22, 1942. N. P. GOSS 2,305,615

SHEET METAL AND METHOD OF MANUFACTURE Filed Jan. 19, 1940 HOT ROLLED STRIP .l57..25Z (3 RE- DUCED 70% OR Mona BY Cow ROLLING Fig.2.

CoLD ROLLED STRIP NORMALIZED CONTINU- OUSLY ABOVE A (e.g.,abou+l700F.) IN RE- DUCING ATMospHERE,& CooLEo RAPIDLY fig 4 INVENTOR Norman P. Go ss 4 wnazmmm euv Patented Dec. 1942 SHEET METAL AND METHOD OF MANUFACTURE Norman P. Goss, Youngstown,

The Cold Metal Process Company,

Ohio, assignor to Youngstown,

Ohio, a corporation of Milo Application January 19, 1940, Serial No. 314,623

3 Claims.

This invention relates to sheet metal, e. g., thin sheet steel such as tin-plate, and a method for manufacturing it.

The standard specifications for tin-plate require that the material have a carbon content of between .10% and .12% and that it be low in me talloids. Special characteristics are required of tin-plate for certain applications. Tin-plate made from rephosphorized steel, for example, is the only material acceptable in the trade for making beer-can ends. The general impression in the tin-plate industry has been that a carbon content higher than the range above-mentioned would cause the tin-plate to exhibit excessive pin-hole porosity, apparently because the surface of the material contains more carbides and the steel is not so clean as the standard low-carbon plate.

I have invented a novel method of making tin plate characterized by a yield point and bendability greater than that of present-day tin-plate, which is acceptable for all applications, thus making it unnecessary for rephosphorized steel, for example, to be specified for tin-plate to be used in making beer-can ends. The preferred practice of my invention comprises cold-rolling hot-rolled steel strip, having a carbon content of from .15% to .30%, to approximately the desired finished gauge. The cold-rolled material is then subjected to a surface decarburization by a treatment similar to normalizing, in a reducing atmosphere. The material may then be coldrolled to gauge and tinned. The resulting product is tin-plate composed principally of .15% to .30% carbon steel but having a much lower percentage of carbon in its surface layers.

The preferred practice of the invention will be described in greater detail hereinafter by reference to the accompanying drawing illustrating the procedure diagrammatically and showing photomicrographs of the finished product. In

the drawing- Fig. 1 is a diagrammatic illustration of the cold-rolling of the hot-rolled strip approximately to gauge;

/ Fig. 2 is a diagrammatic showing of the heattreating or decarburizing operation;

Fig. 3 is a photomicrograph of several thicknesses of the resulting product viewed edgewise; and

Fig. 4 is a view showing a portion of Fig. 3 under greater magnification.

In practicing the invention, I provide a coil iii of hot-rolled steel strip having a carbon content of from .15% to 30% and preferably between .109" and .065" thick. I subject the strip to cold reduction to the extent of or more, as by passing it back and forth through a fourhigh resversing mill ll. Alternatively, the reduction may be effected in a multi-stand continuous cold mill. By the cold rolling, the strip is reduced approximately to the desired finished gauge, e. g., .010", the thickness of a large amount of the tin-plate now being made.

The coil of cold-rolled strip indicated diagrammatically at I2 is next subjected to heat-treatment by passing it through a furnace l3 and a cooling chamber I4 in strand form. The furnace may be heated by any desired means and the cooling chamber I4 is preferably provided with cooling ducts l5. Both the furnace and cooling chamber are preferably provided with a conveyor exemplified by rollers l6, to guide the strip therethrough. A reducing atmosphere is maintained within the furnace and cooling chamber by appropriate means (not shown). I

The furnace I3 is maintained at a temperature such that the strip passing therethrough is heated at least momentarily above the A; point, i. e., about 1700 F. To this end, the speed of travel of the strip through the furnace is appropriately correlated with the length of the furnace and the temperature existing therein. The stripmay be moved through the furnace and cooling chamber by driving the rollers IE or by the tension exerted by a recoiler efiective to dispose the heat-treated strip in the form of a coil ll. On passing from the furnace l3 to the cooling chamber I4, the strip is rapidly cooled and by the time it emerges from the chamber l4, it is below the temperature at which atmospheric oxidation proceeds rapidly.

After being heat-treated, the strip may be further cold-rolled, e. g., according to the disclosure of my Patent 2,165,368, or by ordinary' skin passing. After this final cold rolling, the strip is tinned either in strip form or after being sheared into sheets by known methods and apparatus.

The heat treatment of the strip in a deoxidizing atmosphere causes the surfaces of the material to be partially decarburized, leaving the interior with its original high-carbon'content. This is clearly shown by the photomicrographs in Figs. 3 and 4. Fig. 3 shows several thicknesses of the material of my invention viewed edgewise under a magnification of 100, after etching. This photomicrograph shows that the material has a uniformly fine grain, in general. Fig. 4 shows the appearance of the abutting edges of adjacent thicknesses of the material viewed edgewise under a magnification of 500. This photomicrograph clearly shows the difierence between the character of the surface layers and that of the interior of the strip. The surfaces indicated at I8 are substantially pure ferrite while the center portion i9 of the material is composed principally of sorbite or fine-grained pearlite. The surfaces it are substantially free from sorbite, i. e., to a depth of about two grains. A minimum grain size is desirable, in both the sorbite and ferrite, as the finer the grain, the tougher the material.

The product resulting from the method disclosed exhibits a yield point under the Schopper bend test of from 40 to 50 kilograms per square millimeter and also is capable of from to 8 reverse bends. The high yield point indicates great resistance to deformation while the large number of reverse bends signifies the great ductility of the material. By way of comparison,

the material produced according to the method disclosed in my patent aforementioned exhibited a yield point of about 40 kilograms per square millimeter in the Schopper tester and was capable of about 3 or 4 reverse bends. The bends referred to are those transverse to the direction of rolling since the material is capable of a smaller number of reverse bends in that direction than in the direction of rolling.

By reason of its increased resistance to deformation, the new material makes possible a saving of from 20% to 35% in the weight of a finished container since a thinner gauge of material may be employed without reducing the strength of the container. The material is particularly adapted for sanitary cans or food containers since it successfully withstands the usual hydrogen evolution test which is standard for material to be used in food cans.

In addition to being stronger than ordinary tin plate, as explained above, the product of my invention may be tinned satisfactorily without exhibiting excessive porosity. This is because the decarburization of the surfaces of the material leave them composed of a nearly pure ferrite. The product of my invention when tinned successfully meets all the requirements oi the users in respect to low porosity, besides being stronger than the conventional material.

The tinning of the material effects a change therein which-I refer to as aging.

The heating of the strip to a temperature about 1700 F. and the rapid cooling thereof causes the material to have the maximum strength and ductility after the final cold rolling. The heating is preferably so conducted that the strip is completely recrystallized without any subsequent grain growth.

Because of the increased resistance to deformation characterizing the tin-plate of my invention, it will no longer be necessary for the makers of beer cans to demand that the tinplate to be used for can ends be rolled from rephosphorized steel.

Although I have illustrated and described herein but a preferred practice. of the method of my invention and an embodiment of the novel product, it will be understood that demrtures from either may be made within the spirit of the invention and the scope of the appended claims.

it claim:

1. In a method of making sheet steel of tin plate thickness, the steps including cold rolling a steel strip having a carbon content of from 15% to 30%, heating the cold rolled strip in a decarburizing atmosphere at a temperature sumcient to produce upon cooling a structure of sorbite or fine-grained pearlite in the internal core of the strip and a uniform structure of ferrite on the outer surface of the strip, the core having a carbon content of at least .l5%, the said core carbon content being substantially higher than conventional tin-plate specifications. the outer surface of the strip having a carbon content substantially lower than the core content and thus presenting a commercially tinnable surface, and cold rolling the heat-treated decarburized strip to produce a finalproduct having Schopper values of to kgmJimm. and capable of taking five or more Schopper bends when aged, as by tinning.

2. In a method of making sheet steel of tinplate' thickness, the steps including cold rolling a steel strip having a carbon content of from 15% to 30%, normalizing the seine above the A3 point in a decarburizing atmosphere thereby simultaneously producing an internal core having a carbon content of at least 15% and'a structure of sorbite, and an external commercially tinnable surface having a carbon content below .l5% and a structure substantially of ferrite, and cold rolling the normalized structure to produce a final product having Schopper values of so to 50 kgm. per square millimeter and capable of taking nve or more Schopper bends when aged, as by tinning.

3. As a new product, sheet steel containing from .15% to 30% carbon of tin-plate thicmess with the core and surface structure as defined and produced by the method of claim 1.

NORMAN P. (3088. 

